STARS4Water precipitation deficit for selected river basins in Europe

Precipitation deficit is a common measure for severity of agricultural drought, for example in the Netherlands, but it is also used internationally (Narasimhan, 2005). The precipitation deficit addresses the need for drought monitoring, which was expressed by several river basins, including the Drammen, East Anglia and Danube (Hegdahl, 2023). The precipitation deficit D is defined as the accumulated difference between precipitation P and (potential) evaporation ET. Both P and ET are needed as daily absolute values. As daily evapo-transpiration for future years was not readily available, we decided to calculate it from incoming radiation and air temperature, which are available from Climate and energy related variables from the Pan-European Climate Database derived from reanalysis and climate projections (Copernicus Climate Change Service, 2024). The Priestley-Taylor (1972) formula was used to calculate daily (potential) evaporation. From daily precipitation and (potential) evaporation, we calculate the precipitation deficit by taking the sum of the daily differences (evaporation minus precipitation) from April 1st up to the current date. If the precipitation deficit turns negative, we reset it to zero. For drought severity assessment, we take the maximum value per season (April-September). The precipitation deficit is an accumulated index, which takes into account both the intensity and the duration of water scarcity. Low precipitation and high evaporation over a longer period of time will build up a large deficit. Note that the precipitation deficit is based on the potential evaporation. The actual evaporation will be reduced by limited availability of water in vegetation and the soil. The use of an accumulated indicator implies that a starting date (s=0) is required. In the Netherlands, the starting date is April 1, which is the start of the growing season in this part of the world. For other basins, particularly for southern Europe, the starting date of April 1 may not be appropriate. Moreover, as a result of climate change, droughts may start developing earlier in the year, which would be missed by a precipitation deficit that is starting on April 1. However, this is left for futurestudies. Derived from data from https://cds.climate.copernicus.eu/datasets/sis-energy-pecd降水亏缺（precipitation deficit）是衡量农业干旱严重程度的常用指标，例如在荷兰已得到广泛应用，同时该指标也在全球范围内被采用（Narasimhan, 2005）。多项流域（包括德拉门流域、东盎格利亚流域与多瑙河流域）均提出了干旱监测的需求，而降水亏缺正是为满足这一需求而设计的（Hegdahl, 2023）。 降水亏缺D被定义为降水量P与潜在蒸发量（potential evaporation）ET的累计差值，需获取二者的日度绝对数值。由于未来年份的日度实际蒸散发数据难以直接获取，我们采用由再分析数据与气候预测衍生的泛欧洲气候数据库（Pan-European Climate Database）中气候与能源相关变量（来自哥白尼气候变化服务局（Copernicus Climate Change Service, 2024））中的入射辐射与气温来计算日度蒸散发。本研究采用普里斯特利-泰勒（Priestley-Taylor, 1972）公式计算日度潜在蒸发量。 基于日度降水量与潜在蒸发量，我们通过累加4月1日至当日的每日差值（蒸发量减降水量）得到降水亏缺。若该值为负，则将其重置为0。在干旱严重程度评估中，我们取每个生长季（4月至9月）的最大亏缺值。降水亏缺是一种累计型指标，可同时考量水资源短缺的强度与持续时长：长期少雨且高蒸发的情景会累积形成较大的亏缺。需注意，本研究中的降水亏缺基于潜在蒸发量计算，而实际蒸发量会因植被与土壤的水分供应受限而降低。 累计型指标的使用需要设定初始起始日期（s=0）。在荷兰，起始日期被定为4月1日，这也是该地区生长季的开端。但对于其他流域，尤其是南欧地区，以4月1日作为起始日期或许并不合适。此外，气候变化可能导致干旱在一年中更早发生，而以4月1日为起始的降水亏缺指标将无法捕捉这类早期干旱，但这一问题将留待未来研究。 本研究数据源自https://cds.climate.copernicus.eu/datasets/sis-energy-pecd